There has been provided a vehicle wireless communication system configured to perform vehicle control, such as door locking/unlocking and engine start, in accordance with a wireless signal to be transmitted and received between a vehicle control device mounted on the vehicle and a portable machine carried by a user. Communication methods between a vehicle control device and a portable machine are roughly divided into three types, namely, a polling method, a passive entry method, and a keyless entry method. According to the polling method, the vehicle control device transmits response request signals at predetermined cycles regardless of the position of the portable machine. According to the passive entry method, the vehicle control device transmits a response request signal to the portable machine when a user approaches or touches a door knob. According to the keyless entry method, the portable machine transmits a signal to the vehicle control device when a user operates the portable machine.
According to the passive entry method, when a user carrying the portable machine approaches or touches a door knob, the vehicle control device transmits a response request signal to the portable machine from a plurality of antennas provided at the vehicle, and the portable machine receives the response request signal and replies a response signal including an ID code. The vehicle control device collates ID codes upon receipt of this response signal. If matching is successful, the vehicle control device permits door locking/unlocking or engine start. According to the polling method, when a user carrying the portable machine approaches the vehicle, the portable machine receives a response request signal from the vehicle control device and operation similar to the above case is performed subsequently.
There is committed, however, improper communication of cheating as if the portable machine at a far position were located adjacent to the vehicle using a repeater configured to relay a response request signal from the vehicle control device and a response signal from the portable machine. Such improper communication using a repeater is called relay attack. A malicious third party different from an owner of a vehicle may commit a crime such as a theft by unlocking a vehicle door or starting an engine by means of such relay attack.
There have been devised various security measures against relay attack. For example, JP 2006-342545 A discloses providing a vehicle with a plurality of transmission antennas at different positions as well as providing a portable machine with a plurality of reception antennas having different axis directions. The portable machine detects reception strength of each signal from the plurality of transmission antennas at the plurality of reception antennas, and determines whether or not the communication is relay attack by comparing reception strength ratios among the transmitted signals. Specifically, the communication is determined as relay attack if the reception strength ratios are equal among the plurality of signals. The portable machine does not transmit any response signal in this case so as not to unlock a door.
There have also been devised various techniques of accurately determining the position of a portable machine with respect to a vehicle in order to improve vehicle convenience and prevent malfunction. For example, JP 2014-34787 A discloses detecting electric current flowing to each of a plurality of transmission antennas when an onboard machine supplies the transmission antennas with electric power to transmit a request signal and changing thresholds corresponding to the transmission antennas in accordance with the current values. A portable machine detects reception strength of the request signal and replies the reception strength to the onboard machine. The onboard machine compares the reception strength of the request signal detected by the portable machine and the threshold corresponding to the originator of the request signal so as to determine the position of the portable machine.
Furthermore, JP 5619223 B1 discloses determining whether or not a portable machine is located outside a vehicle or inside the vehicle adjacent to the exterior of the vehicle in accordance with detection areas of a plurality of exterior transmission antennas. A threshold referred to for defining a detection area of an interior transmission antenna is changed depending on whether or not the portable machine is located outside the vehicle or inside the vehicle adjacent to the exterior of the vehicle. The threshold is referred to for comparison with received signal strength (an RSSI value) at the portable machine. The portable machine is determined as being located within the detection areas of the transmission antennas if the received signal strength exceeds the threshold.
The onboard machine and the portable machine have more loads if the processing performed by the onboard machine and the portable machine for security against relay attack is more complicated.
Meanwhile, recent investigation has revealed that a repeater has reception sensitivity much lower than that of a portable machine. When a plurality of onboard transmission antennas transmits a signal as exemplified in JP 2006-342545 A, a repeater may relay a signal transmitted from only one of the transmission antennas. In this case, it is impossible to determine whether or not the communication is relay attack.